That question was asked by two members of the Rem870.com Forum, VMAX has described what part is that:
As stated before it is a magazine cap detent PN #F17451. It fits over a small spring in a hole in your barrel ring. I have a pic for you:
One has detent the other has only a small hole for a detent. Don’t worry it is not completly nessesary. There are other ways to keep your mag nut tight. Use a rubber o ring that fits over the treads of your magazine or a wave washer from a remigton 1100 forend. When installed before you screw on your mag nut they will keep the nut tight. Also replacing the detent and spring is a quick job for your gunsmith. To hold the spring and detent in the barrel ring will have to be staked.